During deposition of a thin film on a substrate in the formation of a thin film photovoltaic device, the thin film material tends to deposit on unwanted areas of the substrate. This unwanted deposition can be particularly present when a vapor deposition process (e.g., a closed space sublimation process) is utilized to form the thin film layer on the substrate. The thin film material can deposit and/or adhere to the side edges of the substrate (i.e., along the perimeter of the substrate) as well as on the face of the substrate. For example, when depositing cadmium telluride onto a photovoltaic substrate to form a photovoltaic absorber layer of a thin film photovoltaic device, cadmium telluride can also deposit onto the side edges of the substrate.
This unwanted deposition of thin film material on the substrate should be removed prior to completion of the device. For example, such thin film materials present on the side edges of the substrate, which are sometimes referred to as the “pencil edge” of a given panel, should be removed prior to sealing of the photovoltaic device. Additionally, the thin film materials on the side edge should be removed to prevent it from exposure or release to the environment once the device is placed into service. For example, when dealing with cadmium telluride, it is desired that all of the cadmium telluride material be encased within the photovoltaic device to prevent release of cadmium into the environment. However, removal of the unwanted thin film material deposited on the side edge can be a delicate process, since the removal process should not otherwise alter or affect the thin film layer(s) on the face of the substrate.
Thus, a need exists for apparatus and methods for cleaning a side edge of a thin film photovoltaic device. In particular, a need exists for apparatus and methods for removing thin film materials from a side edge of a thin film photovoltaic substrate.